566 ORGANS OF RESPIRATION. 



the number of lobes on each side generally corresponds with 

 that of the primary divisions of the bronchi. 



In the cetacea, they present the most firm compact texture, 

 the largest cells, and the least division into lobes ; they are 

 also but slightly lobed in many of the ruminantia, pachy- 

 derma, and even chiroptera, and in some of the larger her- 

 bivora they present no lobular divisions on either side ; thus 

 approaching, in the lower mammalia, to the undivided con- 

 dition of the lungs in birds. The bronchi ramify to a great 

 degree of minuteness before ending in the small vesicles, or 

 terminal air-cells, on the parietes of which the pulmonary 

 capillary vessels are chiefly distributed. The mucous mem- 

 brane, lining all the nasal passages, the eustachian tubes, 

 the larynx, trachea, and bronchial ramifications, is covered 

 with active vibratile cilia, as in inferior vertebrata, and which 

 continue their active movements on detached portions, long 

 after death. Although the lungs of mammalia do not extend 

 beyond the cavity of the thorax, the air is admitted in some 

 chiroptera, as first shown by Geoffroy in the nycteris, into 

 large cellular sacs surrounding the trunk of the body, be- 

 tween the skin and the muscles, especially of the back and 

 sides in the common plecotus, which assists in bracing the 

 muscles, and probably in aeration, and lightens the body by 

 the retention of heated air, as in birds. The air is intro- 

 duced into these subcutaneous cells of the bats, by a small 

 round aperture, provided with a sphincter, and situated at 

 the bottom of each of the two cheek-pouches. The entrance 

 to the respiratory organs in most aquatic mammalia, as seals 

 and beavers, is protected by the valvular structure of the 

 nostrils, which they can completely close against the water 

 through which they move with rapidity ; and a similar 

 structure is seen in the nostrils of the camels, and some 

 other ruminantia, to protect them against the drifting sands 

 of the deserts. In several cetacea, as the porpise, the 

 nostrils form two elongated moveable muscular sacs, as in 

 some crocodilian reptiles, by extending which to the surface 

 of the water, they can respire freely, while the rest of their 

 body is entirely immersed. 



The trachea of mammalia varies with the length of the 

 neck, in the different species, and is generally shorter and 

 more straight than in birds ; it is nearly of equal calibre, 



